**1. Introduction**

X-ray absorption fine structure (XAFS) spectroscopy allows for the chemical environment of the sample to be investigated by studying X-ray absorption of the sample in the vicinity and above the core level binding energy of the considered atom. A modulation in the X-ray absorption spectrum of an atom carries information about its physical and chemical states. A near-edge X-ray absorption fine structure (NEXAFS) spectroscopy is typically performed in the energy range from a few eV below the absorption edge of the investigated atom to, typically, 20–30 eV above the absorption edge. It is a well-established technique for the characterization of chemical and environmental compounds [1], including organic materials (composed of carbon, oxygen, and nitrogen) that exhibit absorption edges in the soft X-ray (SXR) spectral region, λ = 0.1–10 nm wavelength. The SXR NEXAFS yields information about elemental composition through the observation of the spectral features in the vicinity of the X-ray absorption edge [2], for studies of the intermolecular bond structure of polymers [3] and saccharides [4], or for obtaining polymer fingerprint of the material [5]. NEXAFS was also used to study the functions of low dimensional nanostructures [6], to investigate liquids [7] and nanomaterials [8], or probing electronic and chemical states of battery materials [7,8].

NEXAFS spectroscopic investigations are usually performed using synchrotron radiation (SR) facilities, which provide radiation of very high brilliance and intensity over a broad wavelength/energy range in the X-ray spectral region. However, for SR NEXAFS spectrum acquisition a wavelength//energy scanning approach is used, which is time-consuming and does not allow for time-resolved studies. A concise overview of this can be found in the article, in which motivation for an easy accessed complimentary NEXAFS technique based on a compact X-ray source was pointed out [9]. Several NEXAFS systems, operating in the SXR range, have been developed using laboratory laser plasma light sources driven with picosecond and nanosecond lasers [10–13]. These systems were used in the studies of various materials in vacuum [11–13], while the NEXAFS system based on the source driven with a picosecond laser was used in the investigations on photo induced phase transitions studies [14]. More recently, high order harmonic generation (HHG) sources were also used for soft X-ray spectroscopy [15] reaching even the "water window" spectral range (λ = 2.3–4.4 nm) [16]. Most of the studies, however, required relatively long, multi-pulse exposure to acquire a single NEXAFS spectrum [12], which may become an obstacle to investigating biological specimen or samples that change with time. An example of this is a single-gas jet laser-plasma SXR source employed for NEXAFS experiments [17], in which a very long exposure time, reaching up to ten thousand pulses [18], was necessary to reach sufficient signal to noise ratio to obtain a single NEXAFS spectrum. Such approach discards the possibility for high throughput measurements. To overcome this limitation, a single pulse (single-shot) has to be used for NEXAFS spectrum acquisition. A single-shot NEXAFS has been demonstrated recently using a laser plasma light source based on a solid target [19]. However, solid targets that are known to produce debris associated with laser ablation products, which is a highly undesirable effect. Moreover, the design of the spectrometer, including two separate off-axis zone plates, may be prone to mechanical, vibration instabilities, errors in the alignment of the sample and reference spectra for two spectra acquired separately and integration errors in the minute curvature of lines of equal energy in the spectra obtained using off-axis zone plates.

In this paper, we demonstrate a single-shot NEXAFS experiment with the use of a laser plasma light source, based on a double stream gas puff target, which injects two gasses into the laser-matter interaction region, to improve the overall photon yield from such produced plasmas [20]. The target was irradiated with modest (a few joules) energies of the laser pulses. In the gas puff target, the inner gas was chosen for a specific elemental emission, while the outer gas that surrounds the inner gas decreases the density gradient of the inner gas in the direction of the nozzle axis. This significantly increases the target density in the interaction region and allows to obtain higher extreme ultraviolet (EUV) and SXR yields at more modest pumping conditions. Moreover, the gaseous target does not have a problem with a debris production.

Thus, in this work, we demonstrate single-shot NEXAFS measurements on the thin organic samples with the laser plasma SXR source employing a double stream gas puff target. The SXR emission from krypton/helium plasma, allowed one to perform NEXAFS with a 1.3 ns exposure time. As a proof of principle, a 1 μm thick polyethylene terephthalate (PET) and L-ascorbic acid samples were used. Optical density spectra of both samples were obtained with a single SXR pulse exposure and composition of the PET sample was evaluated to confirm the applicability of laser plasma source, based on a double stream gas puff target, to NEXAFS measurements, obtaining a useful single-shot signal.

As a result, a NEXAFS system was developed, based on 10 J, 1 ns Nd:YAG laser system. In this approach, a simultaneous acquisition of reference and sample spectra was possible, through a specially designed SXR spectrometer equipped with long entrance slit. Such construction facilitates the much more accurate acquisition of the spectra, which are independent of source energy fluctuations as well as mechanical instabilities of the system. The spectral resolution of this compact system is comparable

with early synchrotron-based works. In the following sections, the details about this system will be presented and discussed.
